Tech Intelligence
The Touchy-Feely Side of Imaging Nanoscale—or smaller—structures plus other features of
by Mike May AL
AL materials can be analyzed with today’s technology
It’s not just what you can see in microscopy, but what you can touch and feel. In the early 1980s, two new kids on the imaging block—atomic force microscopy (AFM) and scanning tunneling microscopy (STM)— enabled exactly that. These techniques, known in combination as scanning probe microscopy (SPM), use a probe, position sensors, and extensive signal-processing instrumentation to precisely feel their way around a sample. Briefly, the probe interacts with a surface, and instru- mentation records the data on interaction strength, such as the force on the probe or the current flowing between the probe and the surface. From such data, scientists can determine and view a material’s topology, structure, and composition—often at the nano- or even atomic scale.
In an excellent review of the field,1 Institute for Functional Imaging of Materials2
Sergei V. Kalinin, director of the at Oak Ridge National Lab-
oratory in Tennessee, and his colleagues wrote that “the relatively low cost and ease of use of SPMs made them the workhorse of nanoscience.” Today, scientists use SPM to analyze and control materials—sometimes, atom by atom.
“Advances in this technology can do fascinating things,” says Thomas Mueller, director of product management at Bruker Nano Surfaces (Santa Barbara, CA). “AFM is used extensively for electrical characterization in such hot fields as batteries, semiconductors, and energy research, including studies of organic photovoltaics.”
Exploring electromechanics
Often in science, new technologies can bring a fresh perspective to well-studied fields, and that is the case with SPM and electromechan- ics. In the 18th century, Italian natural philosopher Luigi Galvani made muscles twitch in dead frogs by stimulating them with electricity—that is, an electrical input created a mechanical output. This observation was one of the most seminal moments in the history of physics, giving rise to the modern theory of electricity and entering modern language (“galva- nize”). It is also the first known example of an electromechanical study.
Over the centuries, many other biologists explored the world with reac- tions to electricity—often on increasingly fine scales. Rather than bluntly stimulating a frog leg, neuroscientists developed tools to inject current into single nerve cells and then record the outcome, such as triggering contraction in specific muscles. However, in the world of physics, elec- tromechanics remained less explored and was confined mostly to the relatively small subset of materials known as piezoelectrics, since large
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voltages are necessary to produce a measurable response. For example, thousands of volts need to be applied to a piece of quartz to make it expand or contract by even two nanometers.
With SPM, the measurable scale dips deep into the nanoworld. In describing electromechanics, Kalinin says, “The effect is widespread— almost all materials have some electromechanical coupling, but it’s usually very weak.” For example, applying one volt to a material might create one picometer of deformation, which is one-billionth of a meter or about one-twentieth of the diameter of a hydrogen atom. That’s not much movement, but Kalinin says electromechanics is very strongly related to properties of the material, such as the orientation of the crys- talline axes or how atoms move in it.
This phenomenon can also be used to great effect in modern applica- tions, such as the molecular machines that earned Jean-Pierre Sauvage (University of Strasbourg, France), Sir J. Fraser Stoddart (Northwestern University, Evanston, IL), and Bernard Feringa (University of Groningen, The Netherlands) the Nobel Prize in Chemistry 2016. While these ma- chines can be synthesized, SPM opens the pathway to probe them one by one and potentially assemble them in more complex structures.
At the Institute for Functional Imaging of Materials at Oak Ridge National Laboratory in Tennessee, Sergei V. Kalinin and his colleagues apply scan- ning probe microscopy to many problems and develop new ways to share data. (Image courtesy of Oak Ridge National Laboratory, U.S. Department of Energy.)
JUNE/JULY 2017
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